Abstract
We present a three-dimensional hydrodynamic-biogeochemical model of a wave-driven coral-reef lagoon system using the circulation model ROMS (Regional Ocean Modeling System) coupled with the wave transformation model SWAN (Simulating WAves Nearshore). Simulations were used to explore the sensitivity of water column carbonate chemistry across the reef system to variations in benthic reef metabolism, wave forcing, sea level, and system geomorphology. Our results show that changes in reef-water carbonate chemistry depend primarily on the ratio of benthic metabolism to the square root of the onshore wave energy flux as well as on the length and depth of the reef flat; however, they are only weakly dependent on channel geometry and the total frictional resistance of the reef system. Diurnal variations in pCO2, pH, and aragonite saturation state (Ωar) are primarily dependent on changes in net production and are relatively insensitive to changes in net calcification; however, net changes in pCO2, pH, and Ωar are more strongly influenced by net calcification when averaged over 24 hours. We also demonstrate that a relatively simple one-dimensional analytical model can provide a good description of the functional dependence of reef-water carbonate chemistry on benthic metabolism, wave forcing, sea level, reef flat morphology, and total system frictional resistance. Importantly, our results indicate that any long-term (weeks to months) net offsets in reef-water pCO2 relative to offshore values should be modest for reef systems with narrow and/or deep lagoons. Thus, the long-term evolution of water column pCO2 in many reef environments remains intimately connected to the regional-scale oceanography of offshore waters and hence directly influenced by rapid anthropogenically driven increases in pCO2.
Highlights
Rising levels of atmospheric CO2 are expected to continue decreasing seawater pH and carbonate mineral saturation states across the world’s oceans through a process commonly referred to as ‘ocean acidification’ [1,2,3]
Our results combined with data in the literature allow us to conclude the following: Firstly, most of the changes in carbonate chemistry occur during the transit of water across the reef flat, these changes can be further augmented by high rates of metabolism in shallow lagoons
Changes in carbonate chemistry are as sensitive to the combined length and depth of the reef flat as they are on the ratio of metabolic to wave forcing; they are much less sensitive to variations in channel morphology and to the overall frictional resistance of the reef system
Summary
Rising levels of atmospheric CO2 are expected to continue decreasing seawater pH and carbonate mineral saturation states across the world’s oceans through a process commonly referred to as ‘ocean acidification’ [1,2,3]. In a seminal review of reef community carbon metabolism, Kinsey 1985 [39] found that rates of daily benthic community gross production (P), respiration (R), and net calcification (Gnet) for entire reef flats tended to cluster around typical or ‘standard’ values of 580, 580, and 110 mmol C m22 d21, respectively. These rates were found to be largely independent of latitude and longitude (at least across the Indo-Pacific) despite seasonal and spatial variations in light, ocean sea surface Site P R Gnet.
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